1. Field of the Invention
The present invention relates to a reflective (catoptric) imaging optical system, an exposure apparatus, and a method for producing a device. More specifically, the present invention relates to a reflective imaging optical system preferably useable for an exposure apparatus which uses, for example, the EUV light (EUV light beam) and transfers, onto a photosensitive substrate, a circuit pattern on a mask for example in accordance with the mirror projection system.
2. Description of the Related Art
Attention is directed to an EUVL (Extreme UltraViolet Lithography) exposure apparatus which uses the EUV (Extreme UltraViolet) light having a wavelength of, for example, about 5 nm to 40 nm as an exposure apparatus to be used for producing semiconductor elements, etc. In a case that the EUV light is used as the exposure light (exposure light beam), any usable transmissive optical material and any useable dioptric optical material are absent. Therefore, a reflection type mask is used, and a reflective optical system (optical system constructed of only reflecting members) is used as a projection optical system.
Conventionally, it has been suggested that a reflective optical system, which has an entrance pupil disposed on a side opposite to the optical system with an object plane intervening therebetween, is used as a reflective imaging optical system applicable to a projection optical system of an EUV exposure apparatus, in place of a reflective optical system which has an entrance pupil disposed on a side of the optical system with an object plane intervening therebetween (see, for example, U.S. Pat. No. 6,781,671). In the following description of this specification, the “reflective imaging optical system having the entrance pupil disposed on the side of the optical system with the object plane intervening therebetween” is referred to as “reflective imaging optical system of the near pupil type”, and the “reflective imaging optical system having the entrance pupil disposed on the side opposite to the optical system with the object plane intervening therebetween” is referred to as “reflective imaging optical system of the far pupil type”. In other word, the former is also referred to as “reflective imaging optical system of the normal pupil type” and the latter is also referred to as “reflective imaging optical system of the opposite pupil type”.
The reflective imaging optical system of the far pupil type disclosed in U.S. Pat. No. 6,781,671 is constructed of eight pieces of mirrors. Not only in the EUV exposure apparatus but also in an exposure apparatus of general type, there is a demand to increase the numerical aperture of the projection optical system so as to improve the resolving power. In order to increase the numerical aperture of the reflective imaging optical system, it is necessary to increase a parameter for correcting the aberration by increasing the number of mirrors constructing the optical system. However, increase in the number of the mirror leads to increase in the light loss (optical loss), which in turn lowers the throughput of the apparatus.
The present invention has been made taking the foregoing problems into consideration, an object of which is to provide a reflective imaging optical system of the far pupil type as a reflective optical system which is applicable, for example, to an exposure apparatus using the EUV light and which has a large numerical aperture. Further, an object of the present invention is to provide a reflective imaging optical system of the far pupil type in which the aberration is corrected satisfactorily. Still another object of the present invention is to perform the projection exposure at a high resolution while securing a large resolving power by using, for example, the EUV light as an exposure light, with the application of the reflective imaging optical system of the far pupil type of the present invention to a projection optical system of an exposure apparatus.